#define  _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
    double eps, a = 1, sum = 1;
    scanf("%le", &eps);
    for (int i = 1; a > eps; i++)
    {
        a = (a * i) / (1 + 2 * i);
        sum = sum + a;
    }
    printf("PI = %.5lf", 2 * sum);
    return 0;
}